Rebuilt engine knocks after getting hot? Please help

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CraigKennedy
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Rebuilt engine knocks after getting hot? Please help

Postby CraigKennedy » Sat Aug 20, 2016 8:04 pm

Hey yall, a few months back i finished my 1957 cub loboy restoration. I had the engine rebuilt and rebored .30 over i reassembled evetything myself and torqued everything by the manual.

Problem is, months ago after i got everything back together after running it at high idle it would start knocking. I would cut it off and let it set and it would go back to running like a top. I took everything back apart and couldnt find anything wrong. So i reassembled again, and again it started knocking. Each time it did this it ran for about 30-45 minutes at high idle in neutral.

Well I kept on running it 4 or 5 times with the same deal going on.

Eventually i ran the tractor at hight idle for 4 hours and IT DIDNT KNOCK. Problem solved? I thought so..
I ran the cub for around 16 hours at high idle and it ran like a top again. No knocking at all.

Well today I decided to ride it down the road and back at high idle in 3th gear. I drove about a mile and a half all together and when i got back in the yard, wouldnt you know it.... Started knocking...
It knocked for maybe 5 seconds or so and cut off. I tried to crank it but the starter wouldnt spin it, like it was in a bind.
I messed with the starter for a seco d and it cranked right back up.


Does anybody know what this could be? The starter not being able to spin the engine makes me think compression is killing it- valves out of adjustment-

But I cant be sure, I'm out of ideas and dont know what else it could be.
Thank all of you for help
Craig

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Glen
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Re: Rebuilt engine knocks after getting hot? Please help

Postby Glen » Sat Aug 20, 2016 8:18 pm

Hi,
I would check the timing with a timing light. It could be advanced too far, making the sometimes knocking sound. :)

CraigKennedy
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Re: Rebuilt engine knocks after getting hot? Please help

Postby CraigKennedy » Sat Aug 20, 2016 8:30 pm

Glen, i did line up the timing marks when i was reassembling the engine. But would it cause it to knock randomly like that?
Ill try to check it tomorrow- do you line up the first or the second notch on the pulley with the timing pointer?

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Re: Rebuilt engine knocks after getting hot? Please help

Postby Lee_Petrie » Sat Aug 20, 2016 8:59 pm

Hello,
Just a guess with seizing and engine knocking on a fresh engine rebuild , have you checked piston ring clearance ( end gap clearance ) or piston skirt to newly bored wall for at least a min. clearance ? I had a Cushman engine seized up , when it got hot , with too little skirt to wall clearance , then work just fine when it cooled down !
Just a thought , Lee Petrie , Joliet, IL.

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Re: Rebuilt engine knocks after getting hot? Please help

Postby CraigKennedy » Sat Aug 20, 2016 9:04 pm

When i picked the engine up from the machine shop i did ask about the ring gap and if the cylinders needed to be honed or not, the machinist told me that it was everything was set right and that it did not have to be set. I took his word for it..

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Glen
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Re: Rebuilt engine knocks after getting hot? Please help

Postby Glen » Sat Aug 20, 2016 9:08 pm

Hi,
The starter not able to turn the engine when it is hot CAN be from the timing too far advanced, not sure if it is without being there.
The timing should be checked when the engine is running, after a rebuild, or a tune up, with a timing light.
If your timing light is 12 volts, hook it up to a 12 volt battery, it is brighter than on 6 volts.

Here is a page from the Cub service manual. This is written for a Battery Ignition Unit.
Set the timing at the 2nd mark, which is number 1 in the pic, TDC, at low idle speed, be sure the engine runs as slow as it is supposed to. Then the timing should advance to the 1st mark when the engine is at full speed. It should advance part way to the 1st mark at speeds between idle and full speed, the advance should work right for the engine to run good.

http://www.farmallcub.info/manuals/gss- ... 001-43.jpg
Last edited by Glen on Sat Aug 20, 2016 9:14 pm, edited 1 time in total.

CraigKennedy
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Re: Rebuilt engine knocks after getting hot? Please help

Postby CraigKennedy » Sat Aug 20, 2016 9:13 pm

Glen, thank you I will try to check it sometime tomorrow or in the next few days and i will post any findings

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Re: Rebuilt engine knocks after getting hot? Please help

Postby Randy57 » Sat Aug 20, 2016 10:30 pm

Hello If timing checks to be okay and you chance running the engine again to the point of stalling,you might be able to remove the spark plugs and with a hand crank see if the engine will turn freely.If you try this be sure to protect hands from hot spark plugs and if you are as slow as I am these days the engine may be cooled down by the time the plugs are removed.LOL Just a thought Randy

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Re: Rebuilt engine knocks after getting hot? Please help

Postby Eugene » Sat Aug 20, 2016 11:09 pm

Curious. I've read the initial post half a dozen times. No engine knock until running engine for 30 to 45 minutes. The engine locked up is a problem, confusing. Engine should be fully warmed up in 10 minutes or less, so the lock up is very curious.

First thought is a failing coil causing a miss fire. Second thought is that the distributor's mechanical advance is sticking - this may be the cause of the apparent engine lockup.

Lub the mechanical advance. Oil the wick under the rotor. Cheap try. Next cheap try is to replace the coil with a known good one.
I have an excuse. CRS.

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Re: Rebuilt engine knocks after getting hot? Please help

Postby CraigKennedy » Sun Aug 21, 2016 8:41 am

Eugene, thank you i will try replacing the coil also

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Re: Rebuilt engine knocks after getting hot? Please help

Postby tst » Sun Aug 21, 2016 11:14 am

If I read all this correctly,it knocks then seizes up after the rebuild,I have seen this before, when the engine was bored not enough clearance between the pistons and cylinder bore so when the motor heats up the piston expands with the heat which will make it 1st knock and then seize up as the piston grows, when the engine cools off the piston shrinks and it runs again, the fix is to tear it down an measure the bore and pistons to verify the problem, if you keep running it like this it will score the cylinders and pistons and it will need another rebuild

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Re: Rebuilt engine knocks after getting hot? Please help

Postby Eugene » Sun Aug 21, 2016 11:32 am

tst wrote:If I read all this correctly,it knocks then seizes up after the rebuild,I have seen this before, when the engine was bored not enough clearance between the pistons and cylinder bore so when the motor heats up the piston expands with the heat which will make it 1st knock and then seize up as the piston grows, when the engine cools off the piston shrinks and it runs again, the fix is to tear it down an measure the bore and pistons to verify the problem, if you keep running it like this it will score the cylinders and pistons and it will need another rebuild
Possible.
CraigKennedy wrote:Eventually i ran the tractor at high idle for 4 hours and IT DIDN'T KNOCK. Problem solved? I thought so..
I ran the cub for around 16 hours at high idle and it ran like a top again. No knocking at all.
Craig said that he tore down the engine again after the rebuild and didn't find a problem.
Taking the poster at his word. 16 hours run time would normally rule out piston skirt expansion causing the seize up due to engine temperatures.

Just continuing the discussion.
I have an excuse. CRS.

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John *.?-!.* cub owner
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Re: Rebuilt engine knocks after getting hot? Please help

Postby John *.?-!.* cub owner » Sun Aug 21, 2016 1:14 pm

did you by chance use domed pistons? If so there may not be quite enough head clearance when everything warms up.
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CraigKennedy
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Re: Rebuilt engine knocks after getting hot? Please help

Postby CraigKennedy » Sun Aug 21, 2016 5:25 pm

John, no they are aluminum flat tip pistons

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Re: Rebuilt engine knocks after getting hot? Please help

Postby CraigKennedy » Sun Aug 21, 2016 5:55 pm

Update guys: i got ahold of a timing light and checked my timing.
The TDC mark was about 1/4" above the timing pointer, i loosened the distrubtor and turned it maybe a quarter inch clockwise, now the TDC is lining up with the pointer.
Im letting it run at high idle right now, maybe for the next 30 minutes and see what it does. Will keep posted i may have time to drive it around wednesday and see if this has fixed it.
Will keep yall updated
Thanks again for everybodys help


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